Nano-level evaluation of kerogen-rich reservoir rock

ABSTRACT

Nano-level evaluation of kerogen-rich reservoir rock is described. A nano-scale beam is formed from kerogen-rich reservoir rock. The nano-scale beam includes reservoir rock and kerogen having polymeric properties. A maximum dimension of the nano-scale beam is at least 100 nanometer (nm) and at most 1000 nm. A tension test is performed on the nano-scale beam. The tension test is imaged using a transmission electron microscope (TEM). A material parameter of the kerogen in the nano-scale beam is determined based on results of the tension test and images obtained responsive to the imaging.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of and claims priority toU.S. application Ser. No. 15/866,634, filed on Jan. 10, 2018, which is acontinuation of U.S. application Ser. No. 15/250,551, filed on Aug. 29,2016 and issued as U.S. Pat. No. 9,869,649, which claims the benefit ofpriority to U.S. Provisional Application Ser. No. 62/213,752, filed onSep. 3, 2015. The contents of each of the foregoing applications ishereby incorporated by reference in entirety.

TECHNICAL FIELD

This disclosure relates to hydraulic fracturing, for example, ofhydrocarbon reservoirs.

BACKGROUND

Unconventional hydrocarbon reservoirs are reservoirs with trappedhydrocarbons (for example, oil, natural gas, or combinations of them) inwhich the hydrocarbon mobility is limited. Extraction of hydrocarbonsfrom such reservoirs typically involves increasing the mobility of thehydrocarbons, for example, by hydraulic fracturing. In hydraulicfracturing, a fracturing fluid (for example, proppants and one or morechemicals in an aqueous or non-aqueous base fluid) is flowed through thehydrocarbon reservoir. The fracturing fluid fractures the reservoir rockto increase mobility of the trapped hydrocarbons. Some unconventionalreservoirs include an organic material called kerogen intertwined withthe rock matrix.

SUMMARY

This disclosure relates to nano-level evaluation of kerogen-richreservoir rock.

Certain aspects of the subject matter described here can be implementedas a method. A nano-scale beam is formed from kerogen-rich reservoirrock. The nano-scale beam includes reservoir rock and kerogen havingpolymeric properties. A maximum dimension of the nano-scale beam is atleast 100 nanometer (nm) and at most 1000 nm. A tension test isperformed on the nano-scale beam. The tension test is imaged using atransmission electron microscope (TEM). A material parameter of thekerogen in the nano-scale beam is determined based on results of thetension test and images obtained responsive to the imaging.

In another aspect combinable with any of the other aspects, the materialparameter of the kerogen in the nano-scale beam can includes a tensilestrength of the nano-scale beam.

In another aspect combinable with any of the other aspects, the tensiontest is a cantilever test. To perform the cantilever test, a force ofthe order of micro-Newtons is applied on a free-end of the nano-scalebeam. To determine the material parameter, a bending of the cantileverresponsive to force is measured.

In another aspect combinable with any of the other aspects, the force isapplied at a rate of displacement of substantially between 1 nm/s to 100nm/s.

In another aspect combinable with any of the other aspects, the load isapplied until the nano-scale beam fails.

In another aspect combinable with any of the other aspects, heat isapplied to the nano-scale beam while performing the cantilever test.

In another aspect combinable with any of the other aspects, the forcecan be a cantilever force applied using a nano-indenter. To perform thecantilever test, the heat is applied to the nano-indenter, and thecantilever force is applied using the nano-indenter while applying heatto the nano-indenter.

In another aspect combinable with any of the other aspects, to applyheat to the nano-scale beam, the heat is applied directly to thenano-scale beam and to the nano-indenter.

In another aspect combinable with any of the other aspects, an effect ofthe heat applied to the nano-scale beam on the material parameter of thekerogen in the nano-scale beam is determined.

In another aspect combinable with any of the other aspects, a mechanicalproperty profile of the kerogen-rich reservoir rock is determined basedon the effect of the heat applied to the nano-scale beam.

In another aspect combinable with any of the other aspects, thenano-scale beam includes multiple stacked shale bedding planes. Thetension test is performed either parallel to or perpendicular to theplurality of stacked shale bedding planes.

In another aspect combinable with any of the other aspects, to performthe tension test parallel to the multiple stacked shale bedding planes,tension is applied in a direction that is perpendicular to a directionin which the multiple stacked shale bedding planes are stacked.

In another aspect combinable with any of the other aspects, to performthe tension test perpendicular to the multiple stacked shale beddingplanes, tension is applied in a direction that is parallel to adirection in which the multiple stacked shale bedding planes arestacked.

Certain aspects of the subject matter described here can be implementedas a method. A beam is formed from a cement mixture that includes cementand a polymer. A maximum dimension of the beam is at most 1000micrometer (μm). A mechanical experiment is performed on the beam. Themechanical experiment includes a tension test or a compression test. Themechanical experiment is imaged using a scanning electron microscope(SEM) or a transmission electron microscope (TEM). A material parameterof the polymer in the beam is determined based on results of themechanical experiment and images obtained responsive to the imaging.

In another aspect combinable with any of the other aspects, themechanical experiment is the tension test, and the material parameter ofthe polymer in the beam is a tensile strength of the beam.

In another aspect combinable with any of the other aspects, the tensiontest is a cantilever test. To perform the cantilever test, a force ofthe order of micro-Newtons is applied on a free-end of the beam. Todetermine the material parameter, a bending of the cantilever ismeasured responsive to force.

In another aspect combinable with any of the other aspects, beforeforming the beam, the cement mixture is treated with an organic additiveconfigured to alter properties of the polymer in the cement mixture. Aneffect of organic additive on the polymer in the mixture is determinedbased on the material parameter of the polymer in the beam.

In another aspect combinable with any of the other aspects, the maximumdimension of the beam is at least 100 nanometer (nm).

In another aspect combinable with any of the other aspects, the beam isformed using a focused ion beam.

The details of one or more implementations of the subject matterdescribed in this specification are set forth in the accompanyingdrawings and the description below. Other features, aspects, andadvantages of the subject matter will become apparent from thedescription, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an image of porous fibroblast and porous collagen.

FIG. 1B is an image of porous kerogen.

FIG. 2A is a schematic of an example testing apparatus inside an SEM.

FIG. 2B is a flowchart that shows an example process for determiningproperties of a micro-scale rock sample.

FIGS. 3A-3I are scanning electron microscopy (SEM) images ofkerogen-free shale.

FIG. 4 is a schematic of complex layering of illite and kerogen inkerogen-rich shale.

FIGS. 5A and 5B are SEM images of an example shale formation includingkerogen-rich shale.

FIGS. 6A-6D are SEM images and a schematic diagram of kerogen-freeshale.

FIGS. 6E-6H are SEM images and a schematic diagram of kerogen-richshale.

FIG. 7 is a plot of stress versus strain and load/unloading small strainYoung's Moduli.

FIGS. 8A and 8B are SEM images of shale.

FIG. 9A is a schematic diagram of a FIB-SEM sample.

FIG. 9B is a SEM image of a FIB-SEM sample.

FIG. 9C is a SEM image of a cantilever load being applied on a FIB-SEMsample.

FIG. 9D is a schematic diagram of a cantilever load being applied on aFIB-SEM sample.

FIG. 10A is a SEM image of a micro-pillar manufactured using the FIB-SEMtechnique.

FIG. 10B is a schematic diagram showing dimensions of a micro-pillar.

FIG. 10C is a SEM image of a micro-pillar on which load is applied.

FIG. 11 shows a load versus displacement curve for four micro-beams.

FIGS. 12A-12H show load versus displacement at multiple time instantsduring progressive cantilever loading of a micro-beam.

FIGS. 13A and 13B are detailed load versus displacement curves showingearly failures with linear loading and rebounding slopes isolated.

FIG. 14A is an SEM image of a cantilever micro-beam KRS with organicrod-like material.

FIG. 14B is a SEM image of Woodford shale.

FIG. 14C is a full load versus displacement curve.

FIGS. 15A and 15B show top views of a cantilever micro-beam with totalbreakage of the granular shale matrix at the support stage.

FIGS. 16A-16D show a numerical modeling of a cantilever micro-beambehavior.

FIG. 17 is a load versus displacement curve showing strain hardeningbefore a sharp snap at failure.

FIGS. 18A-18F show load versus displacement progress between two points.

FIGS. 19A and 19B are two load versus displacement plots ofmicro-cantilever beam tests.

FIGS. 20A-20D show load versus displacement curves and SEM images beforeand after brittle failure of a sample.

FIGS. 21A and 21B show moduli of ruptures of granular shale (T3)compared to kerogen elastomer cross-linked polymer in T1 and T2.

FIG. 22A is a SEM image of a micro-pillar pre-loading overlaid withenergy dispersive X-ray spectroscopy (EDS) map.

FIG. 22B is the EDS of the micro-pillar displayed and superimposed.

FIG. 23A is a plot of stress versus strain in a micro-pillar compressiontest.

FIG. 23B is a SEM image of the micro-pillar after failure.

FIG. 23C is an EDS map of the micro-pillar superimposed showing theintact shear band plane pre-failure.

FIGS. 24A-24F are SEM images of failed micro-beams.

FIG. 25 shows an example of a fracture treatment for a well.

FIG. 26A-26B are schematic diagrams of indenter tips at differentorientations relative to shale bedding planes for a tension andcompression test respectively.

FIG. 27 is a flowchart that shows an example process for determiningproperties of a nano-scale rock sample.

FIG. 28 is a Transmission Electron Microscope (TEM) image 2800 of anano-scale beam prior to failure.

FIG. 29 is a TEM image 2900 of a nano-scale beam after failure.

FIG. 30 is a plot showing load versus displacement curves on multiplerock sample beams.

FIG. 31 is a plot showing load versus displacement curves on multiplerock sample beams.

FIG. 32 is a flowchart that shows an example process for determiningproperties of a cement mixture.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

Unconventional reservoirs such as organic rich shale have been thesubject of micro- and nano-mechanical characterization using theadvances of nanotechnology. Shale and mudstones were tested using anano-indenter while searching for the micromechanical characterizationof shale rocks. One study was interested in GEOGENOMING™ clay andmudstones for applications in wellbore drilling stability and faultgauge micro-mechanics. Another study attempted to relate kerogenstiffness and anisotropy to its maturity for organic rich source shale.In these efforts, indenting at nano- and micro-scales, thus isolatingmineral phases from the kerogen ones, it was concluded that kerogenstiffness is isotropic. Kerogen-free shale (KFS) was found to bestrongly transversely isotropic at nano- and micro-scales. However, thekerogen stiffness and the percent volume phase, vis-à-vis the rest ofthe shale minerals, reduced the shale anisotropy in many instances inultrasonic pulse velocity measurements. These early nano-indentationstudies were attempts to measure the mechanics at the smallest possible“porous unit” of a mudstone rock, that is, attempting to identify whatis the scale of the Representative Elementary Volume, REV, of fluidfilled shale composites. Their shale samples used in these earlyexperiments contained only “trace” levels of organic material, whichmeans the organic matter had little effect on the overall mechanicalresponse (the total clay content was more than 75 wt %).

Further nano-indentation studies were conducted on the organic-richWoodford shale (≤30% clay; 10-18% kerogen) allowing the observation ofthe effects that the kerogen matrix has on the overall mechanicalproperties of KRS, including the effects on elastic and plasticbehavior. The upscaling of poro-mechanical anisotropic parameters of KRSfrom nano-indenter characterization to macro-rock mechanics laboratorymeasurements and to field logging tools has also been the subject ofcertain studies.

Very little light has been shed on how the KRS fails in tension (such asin hydraulic fracturing) or in compression (such as in drilling) at themicro- and nano-scales as well as the effects of the kerogen polymernature and its spatial distribution on the overall shale matrix.Classical rock mechanics testing on KRS in both tension and compressionhave been performed with respect to deposition modes both parallel andperpendicular to the bedding planes of the Woodford shale. However,these ASTM and ISRM standard test methods did not reveal any noveltiesabout the failure mechanisms of the Woodford KRS.

This specification describes loading and failing KRS using micro-beamsand micro-pillars. In some implementations, micron-sized geometries ofpreserved Woodford shale were manufactured via focused ion beam (FIB)under SEM, then loaded to failure via nano-indentation under the SEM. Insome implementations, the loading and failing of KRS using micro-beamsand micro-pillars can be performed in situ within a transmissionelectron microscope (TEM). Manufacturing techniques used to manufacturethe test samples can include, for example, lithographic techniques,reactive ion etching, or other semiconductor manufacturing techniques.The associated forces (loads) in micro-Newtons and failures atdisplacements in the range of hundreds of nanometers have shown the truenature of the failure mechanisms, in compression and tension, of thiscomposite polymer-rich porous material. It was observed that the organicphase in the tensile mode acts like a cross-linked polymer withsubstantial tensile strength, and a very large modulus of rupture whencompared to the brittle behavior of granular shale minerals. Thiscomposite material behavior is not new to our scientific community, butkerogen tensile elastic strength has eluded our community to date. Thistype of behavior in natural material is also observed when measuringbone strength due to the presence of porous collagen/fibroblast ascross-linked material. The collagen/fibroblast porous nature that isembedded in bones, mimic the overall composite behavior in tension, asthe porous kerogen spatially distributed within the KRS in the clay andnon-clay mineral matrix as shown in FIGS. 1A and 1B. Also, organiccontent in bio-composites similarly augment by order of magnitudes thefracture energy of their minerals.

FIG. 2A shows an example test apparatus 250 for determining propertiesof a micro-scale rock sample 258. A nano-indenter 254 is placed within ascanning electron microscope 252. A rock sample 258 is located withinthe nano-indenter 256 and can be watched while experiments are takingplace. The nano-indenter tip 256 can be a variety of shape, for example,hemispherical or flat-bottomed.

FIG. 2B shows an example method 200 for determining properties of amicro-scale rock sample. At 202, a micro-scale beam is formed from akerogen-rich reservoir rock. At 204, a mechanical experiment isperformed on the micro-scale beam. At 206, the mechanical experiment isimaged using a scanning electron microscope (SEM). At 208, a materialparameter of the micro-scale beam is determined based on results of themechanical experiment and images obtained.

This specification also describes a preliminary two-dimensionalnumerical model built in order to model the loading and displacementcurve in the composite shale of one of the micro-beams. The emphasis wason the kerogen volume and its intrinsic characteristics at themicro-cantilever beam support, as observed in-situ, compared to thefracture propagation and the strain softening potential of beams. Thetwo dimensional model did capture the micro-beam load displacement curveand its corresponding modulus of toughness.

The Nano Granular Nature of Shale and its Polymer Kerogen

All shale source rock reservoirs have the major components of non-clayminerals like quartz, feldspar and plagioclase, QFP, clays such asillite, mica, smectite, and finally organic matter such as kerogen, andbitumen where the oil and gas reside. An unconventional shale reservoirwith 5 wt % kerogen vol %) is considered kerogen rich. In thisspecification, all the various types of organic matter described aboveare considered to be components of kerogen, since what is of interest isthe mechanics of failure of the composite organic-rich shale, and notthe stage of maturity of the organic matter or the reservoir potentials.In this nano-/micro-mechanics approach, the isolated contribution ofeach KRS component and the role it plays in the intertwined phenomena ofminerals and kerogen matrices and the different mechanisms of failurewere observed. This specification describes interpretations of theexperimental results and provides a preliminary numerical model based onthe likely percent weight that the interlaced polymer kerogencontributes to the overall shale sample behavior.

Nano-Indentation on Kerogen Free Shale (KFS): An Intrinsic TransverseIsotropic Granular Material

FIGS. 3A, 3D and 3G are SEM images of a first KFS sample. The term “x3”in FIGS. 3A and 3D indicates that the sample is viewed parallel to thebedding plane. The term “x1” in FIG. 3G indicates that the sample isviewed perpendicular to the bedding plane. FIGS. 3B, 3E and 3H are SEMimages of a second KFS sample. FIG. 3B indicates that the sample isviewed perpendicular to the bedding plane. FIGS. 3E and 3H indicate thatthe sample is viewed perpendicular to the bedding plane. FIGS. 3C, 3Fand 3I are SEM images of a third KFS sample. FIG. 3C indicates that thesample is viewed perpendicular to the bedding plane. FIGS. 3F and 3Iindicate that the sample is viewed perpendicular to the bedding plane.The images show sub-micron clay particles ranging between 10 nm and 100nm in thickness in a variety of forms and shapes, ranging from sheetpackages (Shale 1), to wavy flake structures (Shale 2) and highlypressed and crushed sheets (Shale 3).

Nano-indentation has been used to test small shale samples with only“trace” of kerogen present, where the volume percent is too small toalter the mechanical behavior of the shale at any scale. These shalesamples studied contained 75-80 wt % clay. The shale samples were testedboth parallel and perpendicular to their bedding plane with thousands ofload versus displacement curves collected, which led to identifying thenano-scale material volume of anisotropy in non-organic shale. Forexample, a tensile strength in a direction parallel to the bedding planeis equivalent to pulling a composite network along its edges in adirection parallel to a surface of the composite network. In anotherexample, a tensile strength in a direction perpendicular to the beddingplane is equivalent to pulling the composite network along its edges ina direction perpendicular to the surface of the composite network. Theresponse of the composite network to the same tensile force in twodifferent, orthogonal directions is measured. These observationsconcluded that the tested shale shown in FIGS. 3A-3I, are granular innature and their anisotropic nano and micromechanical properties dependson their particle to particle contact, their packing densities, and thevarious stiffness of their mineral properties.

The KFS properties varied from one sample to the next, and the clay andQFP compositions varied along with their respective porosities. Thegranular cohesionless system of earth materials, in particular, withcompaction histories, “memory” and compacted densities, are very complexprocesses when it comes to their mechanical properties. Clay-bearingsedimentary rocks, such as shale, formed under even more complexgeological processes, are mechanistically even more complex. The role oftheir mineral composition in the overall mechanical propertycharacterization has been the subject of many studies. The KFS in theSEM images in FIGS. 3A-3I were nano-indented, in-bedding andperpendicular to bedding, and exhibited clear mechanical anisotropy atthese scales without any effects from organic matter.

The Intertwined View of Kerogen Rich Shale (KRS) as a TransverseIsotropic Composite

Shale anisotropy has been known and modeled in our mechanisticapproaches from early on, as a fluid saturated porous media exhibitingtransverse isotropy likely due to mode of deposition, bedding planes,micro-fractures or micro- and nano-clay shape or both and packingporosity as described in the above section. Experimental results,particularly acoustic measurements, provided early evidence of shaletransverse anisotropy. However, for source rock KRS, the acousticmeasurements have attributed shale anisotropy not only to fractures andbedding planes but also to the presence of kerogen interlayered withillite clay minerals as shown in FIG. 4. Previous research has paved theway for geomechanics anisotropy modeling of shale in wellbore stabilityanalysis, reservoir compaction simulation, and shale laboratory testingcharacterization. However, kerogen could not be definitively pinned asthe culprit for anisotropy at all scales. KFS has shown intrinsicanisotropy and in many instances even higher than KRS anisotropy atmicro and macro scales.

However, when the conceived structure of clay and kerogen combined asshown in FIG. 4, is taken to failure by tensile or compressive forces,it will be extremely hard to imagine let alone to model the variousphases and how will they interact with the rest of the shale matrix.Kerogen as a polymer has intrinsic mechanical properties for elasticbehavior and its own material properties at plastic yield. Thelimitation of an isotropic plastic model to be able to model the plasticyield from nano-indentation of the KRS has been addressed in previousresearch. The anisotropic stiffness parameters and the nature of organicfree and or organic rich shale and their intrinsic transverse isotropyfrom nano- to macro-scales have been addressed in detail elsewhere.

Example of a Shale Formation

This specification describes nano- and micro-scale Woodford KRS taken tofailure in tension and compression. As background, a brief descriptionof the geological setting is provided below.

The Woodford shale formation, deposited during the lower Missisipian andupper Devonian period in an anaerobic marine environment, is foundthroughout the central part of the U.S. Midwest. The formation has longbeen known to be one of the major source rocks of the region, and forthe past decade it has been a great source of energy in gas and oil.Woodford shale has high quartz content as revealed by X-ray diffraction(XRD) analysis, greater than 20% in total porosity, and permeabilitiesranging from 80-40 nano-Darcys. While it is typical of source rock shaleto have kerogen dispersed in its structures, the Woodford showspronounced intertwined kerogen strings shown in two-dimensions whencompared to the overall granular mineral matrix. FIGS. 5A and 5B showvery complex shapes of organic material (kerogen) in the Woodford. TheSEM images of Woodford shale highlight the intertwined nature ofminerals and kerogen (black polymer-like ribbons). The scale of theribbons is tens of micro-meters.

The heterogeneity of the Woodford KRS, like all source shale, is dueamong many reasons, to local non-clay minerals such as quartz, calciteand pyrite, and clay minerals intertwined with kerogen string-shapedcomponents at nano, micro and macro levels. Similar to the multiscalestructure of KFS a complementary KRS multiscale mechanistic structure,based on SEM images, is shown in FIGS. 6A-6H.

FIG. 6A is a macro-level SEM image of kerogen-free shale, for example,porous clay-silt inclusion composite, taken at a scale of greater than10⁻³ m. FIG. 6B is a micro-level SEM image of a portion of thekerogen-free shale shown in FIG. 6A taken at a scale of greater than 10⁻m. FIG. 6C is a sub-micro-level SEM image of a portion of thekerogen-free shale shown in FIG. 6B taken at a scale of greater than10⁻⁷ m. FIG. 6D is a schematic drawing of a portion of the kerogen-freeshale shown in FIG. 6C drawn at a scale greater than 10⁻⁹ m. FIG. 6E isa macro-level SEM image of kerogen-rich shale, for example, layeredcomposite shale with clay/quartz mix (light gray) and organic layers(dark gray), taken at a scale of greater than 10⁻³ m. FIG. 6F is amicro-level SEM image of a portion of the kerogen-rich shale shown inFIG. 6E taken at a scale of greater than 10⁻⁵ m. The image shows kerogenand micro-pores distributed throughout the mineral matrix. FIG. 6G is asub-micro-level SEM image of a portion of the kerogen-rich shale shownin FIG. 6F taken at a scale of greater than 10⁻⁷ m. The image showsnano-porous minerals interwoven with nano-porous organic matter. FIG. 6His a schematic drawing of a portion of the kerogen-rich shale shown inFIG. 6G at a scale greater than 10⁻⁹ m. The schematic diagram showselementary components, namely, clays such as illite, smectite, etc., andorganic molecules, for example, kerogen.

In compiling this micro to macro structure with micro-bedding planes andmicro-fractures shown at level II, the failure mechanisms of suchcomposite are very complex. For example, in tensile loadings, thepolymer and rubber-like kerogen embedded in the shale matrix, at allscales, will augment the tensile rupture (modulus of toughness) of thegranular fractured structure matrix.

Macro-Scale Testing of Shale in Light of Kerogen Content and CompositeNature of KRS

In this section, the data and the macro-scale testing conducted on thesame preserved Woodford is revisited for many details that previouslywere missed since kerogen content, and the composite nature of KRS, wasnot considered in the previous data interpretations. In the previousstudy, only the classical geomechanics approaches were considered withcorresponding mechanical parameters. FIG. 7 shows the loading andunloading up to failure of an ASTM 2″×4″ standard of the preservedWoodford KRS in an unconfined compressive loading configuration. Theunconfined compressive strength value is more than 5000 psi incompression with the large sample deformation close to 0.8% strain. Theaxial and radial stress/strain curves show the slightly plastic yielddeformation starting at the third round of loading/unloading (thestraight dotted line on the axial deformation) eventually masked bypiece-wise partly linear slope to eventually undergo brittle failure.Yet the small strain Young's moduli shown in Table 1 (below) at thethird and fourth cycles were unaffected by the stress yield. Thisbilinear elastic behavior, followed by a brittle failure, is intriguingand is difficult to explain, considering single granular phase behavior.

TABLE 1 Young's Modulus for small strain measurements for Woodford shalesample. Axial Stress Cycling (psi) E (Mpsi) 1250 1.57 2300 1.53 37001.62 5500 1.62

Another observation is that the Young's moduli measured atloading/unloading cycles were more than 50% larger than the overallYoung's modulus of the full testing load range shown in Table 2.

TABLE 2 ASTM measurements. Sample 166-2 to 166-6 Strength (psi) 1800 E(kpsi) 660 □ 0.3

The value of the dynamic Young's moduli calculated from thecompressional and shear waves velocities were 10-15% different from theloading/unloading small strain cycles, thus confirming the granularporous nature behavior of this shale when undergoing compressive smallloads.

Recent data summarizes another large campaign of nano-indentationtesting on these same horizons of the preserved Woodford KRS. The fullsweep of tests on shale samples, both parallel and perpendicular tobeddings, showed that the organic matters have anisotropic stiffness,and much smaller stiffness values than reported previously in the planeparallel to beddings. Recent research indicated that damage may haveoccurred during cutting and polishing, due to heat, altering theinherent kerogen anisotropy, and that the kerogen rebound when load wasremoved and some permanent deformation (plastic) remained as evidencedby the indentation imprint. FIGS. 8A and 8B provide much clearer SEMimages that, illustrating clearly what was called “ . . . indentationinto a highly heterogeneous region,” showing a large percentage of theorganic matter and minerals and being simultaneously indented. FIG. 8Ashows a polished surface with organic material which includes 1 μm-sizeddiameter pyrite framboids, silicate, clays, etc. FIG. 8B shows a similarregion which has been indented. The final area projected after theindent imprint is roughly 450 μm². The preliminary conclusion of theseabove described experiments is that the organic matter in the sourceshale needs to be somehow reinvestigated within the overall framework ofthe porous shale.

Example of an Experiment to Prepare a Kerogen-Rich Shale (KRS) Sample

Focused Ion Beam (FIB)—Scanning Electron Microscopy (SEM) samplepreparation of specific geometries such as micro-pillars andmicro-cantilevers of KRS are described here. In some implementations,four micro-beams and three micro-pillars were milled and prepared forin-situ testing.

Example of Cantilever Testing KRS Micro-Beams Using a Pico-Indenter(PI-85) in the FIB-SEM

A sample with dimensions of 1 cm×1 cm×0.4 cm was cut from a preservedWoodford KRS core. A sharp 90° edge was created by mechanical polishingusing standard silicon carbide paper up to 4000 grit followed bypolishing with 1 μm diamond grit. A Quanta 3D field emission gun (FEG)with FIB-SEM was used to prepare the micro-beams. FIB surface millingwas used to clean the surface for better sample imaging as well as toprepare the desired micro-geometries. Four micro-beams were manufacturedusing the FIB procedure according to the S.G. Roberts method. While thebeams in this experiment were manufactured according to the S.G. Robertsmethod, other manufacturing techniques, such as lithographic techniques,reactive ion etching, or other semiconductor manufacturing techniques,can be used. Each shale micro-beam was shaped by cutting trenches on allthree sides with widths of 20 μm and depths of 10 μm using a 15 nA beamcurrent, resulting in a U-shaped trench. The geometry was then refinedby applying a 1 nA beam current. Afterwards, the sample was tilted to45° along the length axis to shape the cantilever. The base of thecantilever was undercut from both sides using a 3 nA beam current. Theresulting cantilever geometry is shown schematically in FIG. 9A, withthe corresponding SEM images of three of the four micro-cantilever beamsshown in FIG. 9B. It should be noted that this sample size is still wellabove the REV of composite shale. As shown in FIG. 26A and FIG. 26B, thebeams can be manufactured with varying orientations relative to theshale bedding planes, such as perpendicular to the force of the indentertip 256 or parallel to the force of the indenter tip 256. Manufacturingthe beams at the varying orientations can allow studying anisotropy ofthe beams and upscaling the anisotropy to larger KRS samples.

A Hysitron Pi-85 Pico-indenter was used to load the micro-beams underdisplacement control mode, at a rate 10 nm/s. The indenter tip is a flatcircular punch geometry, with a diameter of 5 μm. All loadingexperiments were performed in situ under the SEM, where loading of themicro-cantilever beams continued until failure. The indenter tip wasplaced at the end of the beam, centered along the y-axis as shown in theSEM in FIG. 9C. Load and displacement data were collected in real-time.FIG. 9D is a schematic diagram showing the micro-experiment withdimensions.

During the experiment, a force (micro-Newtons) is applied to the beam orpillar through the nano-indenter tip. As the force is applied, the beamor pillar deforms (meaning the indenter tip is displaced in nanometers).Both the force and displacement are captured by the nano-indentersoftware throughout the experiment. Typically the rate of displacementis controlled (for example, 1-100 nm/s, 5-20 nm/s or other rate ofdisplacement) while the force is applied to such a degree as to maintainthis displacement rate. Because this experiment is performed inside ascanning electron microscope (SEM), the fourth parameter captured(beyond force, displacement, and time) is an SEM image. In fact, the SEMimages are captured throughout the entire loading experiment as a movieof the entire experiment. Finally, additional analysis of the micro-beamand micro-pillar can also be performed with energy dispersive x-rayspectroscopy (EDS) while the sample is inside the SEM. This measurementprovides the chemical (elemental) composition of the sample. It can beperformed pre-loading, post-failure, or in some configurations, duringthe loading.

Earlier, it was illustrated from macro measurements on 2×4″ samples thatthe loading/unloading Young's Moduli differed from the large strainYoung's Modulus by more than 50% but are within 10% of the dynamicmeasurements. Also, the values of Young's moduli obtained bynano-indentation on porous multiphase material are close in value to thesmall strain deformation and to the ultra-pulse velocity measurements.However, when a solid metallic beam with micron-sized dimensions issubjected to loading, there is strong evidence that size effects comeinto play. This phenomenon has been elaborated on and theoreticalresults have been obtained corresponding to an intrinsic length scaleeffects on the overall deflection, w, of a solid micro-cantilever beamwith intrinsic length scale, l_(FE) that is found by calibrating atypical beam thickness with the experimental suite of results. Theexpression relevant to the experiments described here is shown inEquation (1).

$\begin{matrix}{w = \frac{{Px}^{2}\left( {{3L} - x} \right)}{6{E\left( {I + {bhl}_{FE}^{2}} \right)}}} & (1)\end{matrix}$

In Equation (1), I is the moment of inertia for the micro-cantileverbeam prismatic cross section. The length of the cantilever beam runsalong the x-axis, and the position of the indenter tip along that axisis denoted as x. It is assumed that that x=L because they are verysimilar. The parameter E is the Young's modulus, which is a measure ofthe stiffness. It is reported in units of GPa. Equation (1) providedsatisfactory results when used to analyze experimental measurements.However, Equation (1) may need to be modified for a granularmulti-porous structured material intertwined with organic matter. Thediscovery is that when we assumed l_(FE)=0 for Equation (1), where itturns into the expression for the classical theory of beams, themicro-cantilever beam Young's moduli was within 10% error from the onesshown in Table 1. Indeed, results calculated from the classical theoryfor the stiffness expression (Equation (2)) match the nano-indenterresults as well as the small strain loading/unloading of FIG. 7, and thecorresponding ultra-pulse velocity measurements using the compressionaland shear wave velocities.

$\begin{matrix}{E = \frac{{PL}^{3}}{3{wI}}} & (2)\end{matrix}$

Example of Compression Testing KRS Micro-Pillars Using a Pico-Indenter(PI-85) in the FIB-SEM

As shown in FIG. 10A, square micro-pillars with minimal taper weremanufactured in the FIB instrument (Quanta 3D FEG) with successivelylower beam currents (5 nA down to 0.3 nA at 30 kV) to achieve thegeometries shown in FIG. 10B. Alternatively, the micro-pillars can haveother cross-sectional shapes. For example, the micro-pillars can beround. The milling procedure followed the methods of earlier works. Toachieve the square geometry, the sample was tilted by ±2° with respectto the incident ion beam in order to mill the side surfaces of thepillar by grazing incident ions. The aspect ratio (micro-pillar heightdivided by width) was set close to three to one. These dimensions mayvary slightly, eventually, if these tests are to be standardized forporous natural material such as shale. While the beams in thisexperiment were manufactured in the FIB instrument, other manufacturingtechniques, such as lithographic techniques, reactive ion etching, orother semiconductor manufacturing techniques, can be used.

FIG. 10C shows a load being applied to a micro-pillar. A differentHysitron indenter was used to uni-axially compress the micro-pillarsusing a diamond flat punch tip indenter (60° conical, 10 μm diameterflat end) as shown in FIG. 9C. The micro-pillars were loaded at apredetermined displacement rate until failure. FIG. 9B provides aschematic of the micro-pillar with dimensions of b, h, and L and appliedforce P. The compression of the micro-pillar samples under thenano-indenter can be described by the classic compressionalstress-strain relationship.

Micro-Beam Testing

FIG. 11 shows a load v/s displacement curve for four micro-beams. Fourmicron-sized beams were milled to load and fail Woodford shale intensile mode. Each test was performed inside the SEM with a small-scalenano-indenter, and movies of the loading and failure were captured inreal time during the experiment. This unique setup provided not only theability to load and fracture micro-scale KRS structures but also theadvantage of visualizing the initiation of a fracture in the tensilezone, then propagation, and ultimate failure while correlating thesephenomena with the force-displacement plots collected during theexperiments. The load-displacement curves (FIG. 11) show that samples T3and T4 failed in brittle modes while samples T1 and T2 showed plasticdeformation before failure. Samples T1 and T2 showed strain softeningand strain hardening behavior, respectively before ductile failure.Samples T3 and T4 showed brittle failure with little or no yield.

The load-displacement curves captured from loading at the tip of themicro-cantilever beams of equal prismatic dimensions can be compareddirectly. The areas under their respective force-displacement curves areproportional to the energies required to break the beams in a tensilemode (as shown in FIG. 21A and FIG. 21B). The higher the energy, thelower the fracturability of the rock (more ductile). Higher kerogencontent within the beam leads to much larger displacement before failingand thus much higher energy (Energy=work=Force×Displacement).

Elastic Loading in Pre-Yield and Strain Softening in Post Yield

FIGS. 12A-12H show load versus displacement at multiple time instantsduring progressive cantilever loading of a micro-beam. FIGS. 12A-12Hshow four stages of a test with the load-displacement correlated to thein situ real-time SEM pictures of the micro-cantilever beam progressiveloading to failure. FIG. 12A is a load versus displacement curve showingthat a cantilever micro-beam shown in FIG. 12B is continuously loaded upto P=809 μN with a displacement w₁=697 nm in a linear elastic loaddeformation curve. FIG. 12C is a load versus displacement curve thatshows that a sudden drop in stress occurs after point 1. FIG. 12D is aSEM image that shows a crack close to the top of the beam. However, thebeam continues to deflect and soften as the indenter continuously loadsthe tip of the micro-beam to point 2 in FIG. 12C. FIG. 12E is a loadversus displacement curve and FIG. 12F is a corresponding SEM image ofthe cantilever micro-beam showing the development of a complex strainsoftening post yield, and a continuation of fracture propagation towardsthe bottom of the micro-beam. In this frame, the cantilever micro-beamhas totally failed and is almost detached from its support with amaximum deflection, w₃=4499 nm. FIG. 12G is a load versus displacementcurve and FIG. 12H is a corresponding SEM image of the cantilevermicro-beam showing an elastic rebound from point 3 to point 4, and thefinal deformation when the indenter is lifted. The deflection w₃ isgreater than w₄ as Shown by the Dotted Lines in FIG. 12H and isEvidenced by the Displacement Elastic recovery shown in FIG. 12Grelative to FIG. 12E.

FIGS. 13A and 13B show the plot details with various slopes followingfailure progress described with reference to FIGS. 12A-12H, particularlyFIGS. 12C-12H. FIG. 13B shows the linear elastic load and rebound curvesin addition to the step-wise linear strain softening behavior. Thelinear elastic early performance followed by the various slopes in thestrain softening regimes extended the micro-cantilever beam rupture to avery large displacement compared to the 500 nm for the early pure linearelastic deformation. From in-situ visualization, the dashed linerepresents the first major fracture. In other words, the kerogen, afterthat point, was supporting most of the load preventing the beam fromreaching its rupture strength. The rebound slope at the bottom afterstage 3 shows a linear elastic rebound proving that the kerogencross-linked elastomer did not reach its rupture strength, but ratherthat mass of kerogen extended the initial shale granular deformation andfailure by almost 10 times to 809 μN.

FIG. 14A is an SEM image of a cantilever micro-beam KRS with organicrod-like material. FIG. 14B is a SEM image of Woodford shale. FIG. 14Cis a full load-displacement curve. The SEM image in FIG. 14A shows thestring-like kerogen. The SEM image in FIG. 14B shows similar worm-likestrings of kerogen (dark lines). The SEM image in FIG. 14B is taken witha 40 μm scale while the beam in the SEM image in FIG. 14B has a totallength of 22 μm indicating that the polymer-like kerogen can be embeddedin the total length of the beam and way further into themicro-cantilever beam fixed support. FIG. 14C shows the full loadinghistory.

FIGS. 15A and 15B show top views of a cantilever micro-beam with totalbreakage of the granular shale matrix at the support stage. In contrastto granular material failure, the polymer-like string in the KRS keepsthe beam attached to the support after a total tensile failure of themicro-beam. The shale matrix granular failure is clearly broken as shownbelow in FIGS. 15A and 15B; yet, the micro-cantilever beam is stillhanging on after the nano-indenter load was released. This behavior istypical of composite beams such as reinforced concrete beam, or ingeo-grid reinforced site constructions. Post failure analysis showsstrain softening behavior that can be reproduced using numericalsimulation. Since the organic content in these shales, such as kerogen,was never observed in tensile loading or tensile failure to have anyeffects, the constitutive model for mechanical behavior of the kerogenmatrix intertwined with other shale minerals is nonexistent. A twodimensional numerical model mimicking the micro-beam response in thisone test was constructed as described below. This is an attempt toexplore the potential constitutive model for the micro-cantilever beammechanical behavior through matching the force—displacement curve byplacing a percent of volume of organic matter with sustainable tensilestrength characteristics at the support.

Numerical Modeling of Cantilever Micro-Beam Behavior

FIGS. 16A-16D show a numerical modeling of a cantilever micro-beambehavior. FIG. 16A shows the contour of maximum shear strain withtensile yielding indicator. FIG. 16B shows the comparison offorce-deflection curve measured by experiments and numerical modeling.FIG. 16C shows three simulation cases with kerogen content varying from60% to 20%. FIG. 16D shows normalized force-deflection curves from threecases also illustrating the modulus of toughness variation. SEM imagesindicate that the kerogen content is quite rich at the fixed end in Test1 and thus contribute to the bending and the mechanical response of themicro-cantilever. In this initial plane-stress numerical model, the realgeometry of the micro-cantilever beam is used, for example, 21.69 μm inlength and 8.80 μm in thickness. The out-of-plane direction is unity (1μm), but the loading force monitored in the numerical model is scaled by9.49 to account for the thickness of the micro-cantilever beam in thenano-indentation test.

It is observed that the tensile yielding only took place at the fixedend. To simplify the setup in the numerical model, the left column ofelements (“kerogen”) is assigned with strain softening capability whilethe rest of the elements are assumed to be pure elastic material. In thenumerical model shown in FIG. 16A, the left boundary of the model isfixed in both X- and Y-directions. The grid point at the upper rightcorner is loaded in the downward direction at a constant rate of 1×10⁻⁵μm/step. The reaction force and deflection at the loaded grid point aremonitored during the entire course of the simulation. The modelingapproach is to adjust material properties so that the force—deflectioncurve measured in the numerical simulation matches with the measurementin experiment described with reference to FIGS. 12A-12H. The elasticresponse (before the peak) is matched by adjusting material stiffness inthe model. The plastic component is matched by adjusting the strainsoftening curve of the kerogen material. Simulation indicated that aYoung's modulus of 14 GPa in tension and a bilinear tensile strainsoftening (for example, tensile strength is 130 MPa initially, decreasesto 85 MPa at plastic tensile strain of 17% and further drops to theresidual tensile strength of 12 MPa at plastic tensile strain of 70%)seem to give quite a good match, as presented in FIG. 16B. The tensilecrack area developed near the fixed end in FIG. 16A also looks similarto the lab observation. The Young's modulus used in the numerical modelis very close to the values measured for the four micro-beams.

Strain Hardening Before a Sharp Snap at Failure

FIG. 17 is a load versus displacement curve showing strain hardeningbefore a sharp snap at failure. The test described with reference toFIG. 17 was performed in two stages. The first stage showed load/unloadhighlighting relatively elastic behavior with one minor kink observed at3000 μN. But, the micro-beam continued displaying elastic behaviorduring loading up to 3500 μN. The beam was then unloaded, stopping atpoint 2 then loading and unloading again to confirm its elastic linearbehavior. An approximately equal parallel slope to the first loadingcurve and an almost total recovery of the elastic linear displacementwas obtained.

FIG. 17 also shows the second stage, where the same micro-beam wasimmediately reloaded without any disturbance in between stages or evenany lapse of time. The progress of this loading is illustrated in FIGS.18A-18F (frames 1-4 in chronological order). As shown in FIGS. 18A-18F,the fracture has already propagated as the load increased from 3500 to4000 μN across the depth, h, of the micro-beam near the fixed supportyet the load continue to increase reaching above 5000 μN before totalrupture. Passing the threshold of the earlier elastic, a kink issuddenly observed at 3800 μN, and a minute transversal crack shows up onthe beam. Then, the micro-beam recovered shortly to a load value closeto 4050 μN before a substantial failure shown in FIG. 18F (Frame 3) thusdecreasing the load to a little less than 3000 μN shown on FIG. 17 aspoint 3. As the loading continues beyond this point, clear strainhardening behavior is observed, while a major fracture has developed asseen in FIG. 17 (Frame 4). However, the micro-beam continues to gainenergy before the final snap and total failure at point 4 in FIG. 17.This ultimate tensile load is equivalent to the ultimate tensile stress(UTS) which is a value that carries much significance in ourgeomechanics source shale field fracking applications. In fact, the UTSis much more important than the UCS since hydraulic fracturing is atensile mode one crack failure and not a compressive one.

Brittle Failures with Minimal or No Yield

FIGS. 19A and 19B are two load versus displacement plots ofmicro-cantilever beam tests. FIGS. 20A-20D show load versus displacementcurves and SEM images before and after brittle failure of a sample.Micro-cantilever beams T3 and T4 are shown in FIGS. 19A and 19B,respectively, where sharp brittle failures with clear snaps wereobserved during the in situ SEM imaging. The brittle failure isindicative that there are only trace amounts of organic matter at thefixed support of the micro-cantilever beam. As defined early on, “trace”organic matter is an amount that is not enough to alter the mechanicalbehavior. This fact was illustrated in the numerical simulationdescribed above, where reducing the volumetric percentage of kerogen atthe support diminished the strain softening as well as the ultimate loadobserved during elastic loading. The Young's moduli measured at 50% ofthe maximum load in the elastic regime are very close in value, as shownin Table 3 (below); yet their failure loads varied by almost 100%.

TABLE 3 The summary of dimensions and calculated values for each of themicro-beam tests. L b h I P E Test (μm) (μm) (μm) (μm⁴) (μN) (GPa) 121.69 9.49 8.80 539 290 9.1 2 21.37 7.36 9.23 483 1016 30.4 3 24.12 7.259.47 514 353 14.2 4 23.18 7.95 9.94 651 478 13.5

In summary, the four micro-beams showed very interesting behaviorswithin a span of 200 μm in the preserved Woodford KRS. In Table 3, thedimensions of each micro-cantilever beam are summarized to illustratethe difficulty of attempting to obtain exact dimensions for each milledporous micro-beam. The calculated values of the Young's Moduli weretaken at ˜50% from the linear elastic loading span, that is, they werecalculated based on picking up the corresponding load, P, and thedeflection, w, at 50% on the four loading curves.

FIGS. 21A and 21B show moduli of ruptures of granular shale (T3)compared to kerogen elastomer cross-linked polymer in T1 and T2. Themodulus of toughness as the work/energy needed before the total ruptureof the beam is illustrated in FIGS. 21A and 21B. It can be seen that thetwo shaded areas where T3 required ˜10% of the toughness needed to breakmicro-beam T2. A similar comparison can be made between T3 and T1although the magnitude is different. Hydraulic fracturing involves thetensile cracking of this composite KRS formation.

Micro-Pillar Compression Testing

FIG. 22A is a SEM image of a micro-pillar pre-loading overlaid with EDSmap. FIG. 22B is the EDS of the micro-pillar displayed and superimposed.While three micro-pillars were prepared for the testing only one endedup being loaded to failure. EDS has been used to analyzenano-indentation results and to isolate mechanical phases whendescribing Woodford mechanical behavior. In this test, EDS was alsoconducted on the micro-pillar face before testing as shown in FIG. 22A.A pre-existing inclined shear band can be observed across the center ofthe micro-pillar, as evidenced by the orange color which is enlarged inthe superimposed frame shown in FIG. 22B. The top of the pillar has ahigh concentration of silicon and oxygen, indicating silica, the middleband contains aluminum, silicon, potassium, and oxygen indicating clay,while the bottom section has a high concentration of calcium andmagnesium indicating dolomite.

FIG. 23A is a plot of stress versus strain in a micro-pillar compressiontest. FIG. 23B is a SEM image of the micro-pillar after failure. FIG.23C is a EDS map of the micro-pillar superimposed showing the intactshear band plane pre-failure. The result in FIG. 23A shows the variousstages of the stress-strain curve. Initially, a non-linear loadingsection is observed. Then, as the load increases in a linear elasticpart, illustrated on the plot with the straight line extension, beforethe micro-pillar goes into short yield. Then, a load-unload section andeventually failure are observed. This stress strain curve resemblesexactly, in its stages, the uniaxial 2″×4″ Woodford KRS sample describedabove. Even the load and unload part of the curve, shows a higherYoung's modulus, also consistent with the load/unload cycles describedabove. Since the loading/unloading occurred after a short yield, themuch higher Young's modulus could be due to permanent micro-pillarconsolidation or pore/grain compaction. Eventually, the shear band actedas a weak plane along which the micro-pillar sheared during failureshown in FIG. 23B with the superimposed pre-failure EDS map in FIG. 23C.Shear bands often form when granular materials undergo ductile behaviorunder a given loading configurations. Having this pre-existingcondition, the shear failure showed little interference of the kerogenpolymer-like behavior.

The micro-beam in T1 exhibited ductile behavior as shown earlier, and itpost-yielded in a strain softening regime while the ductile behavior ofTest 2 demonstrated strain hardening in post-yield. Meanwhile, themicro-beams in Tests 3 and 4 exhibited brittle failure modes.Determining the reasons for the differences between each of the failedmicro-beams is important to be able to upscale and convert thisunderstanding into predictive tools, when it comes to hydraulicfracturing, wellbore drilling, reservoir optimal productivity, and manyother oil and gas field applications.

FIGS. 24A-24F are SEM images of failed micro-beams. The squares in FIGS.24A and 24C indicate sections expanded in subsequent frames and in FIG.24F. The role of kerogen and other organic matter as cross-linkedpolymer contributing substantial tensile strength to the shalemicro-beam have been observed in the KRS tensile loading experiments. InT1, the beam granular structured totally failed, and yet the kerogenstring rebounded the whole beam back recovering some of it elasticenergy. FIGS. 24A and 24B show that the beam had separated from itssupport; and yet, the tensile elastic string stretched but did notrupture or pull out, but rather was still fully embedded in the beam. Inother words, the kerogen string never reached its full “modulus oftoughness” or total energy needed for rupture while early on in theloading range the granular part of the beam reached its “modulus oftoughness” in tensile loading. The amount of kerogen at the support wasenough to give it the strain softening behavior where the micro-beamacted as a composite material such as reinforced concrete beams.

In T2, the amount of kerogen was way too high and even overwhelming atthe support with little volume of the clay or non-clay granularmaterial. The volume of the organic matter that stayed behind at thesupport is evident by the large cavity left on the micro-beam aftertotal collapse shown in FIGS. 24C and 24D. The volume of the kerogen waslarge enough and stiff enough to carry the micro-beam into a strainhardening post yield type of failure with a huge modulus of toughnesscontributing to the overall shale micro-beam behavior. The progress offailure in T2 reinforced our early hypothesis that the cross-linkedpolymer nature of kerogen and its intertwined structure with thenon-clay and clay mineral matrix is the one holding the granular shalematrix together resulting in large and unexpected values for granularmaterial in tensile failures. Also, the fracture has totally developedacross the depth of the beam, yet the beam continued taking more loadand exhibiting strain hardening until total failure.

This work sheds new light on the composite nature of kerogen-rich shale.It showed that the composite nature of the organic rich shale hastensile strength characteristics that are relevant. An obvious questionis, “why for the past decade or so in rock mechanics testing we did notpick up on the tensile attributes of this KRS shale or any other sourcerock formation?” The answer is simply that these tensile characteristicsof polymers are easily masked in the ISRM standard testing methods formacroscale geo-mechanics material characterization such as the Braziliantest and other approved tensile strength measurements for rocks. Thesetests were never designed to isolate or measure the tensile strength ofpolymers. This natural cross-linked polymer component, kerogen, with itstensile characteristics was not known previously to contribute to thetensile strength of any known rock loaded in tension. Now that theorganic rich source shale formations are loaded under tensile forces,for example, Mode One crack opening and crack propagation, the UTS ofthe organic components is of paramount importance to successfullyengineer our lab and field applications

Example of a Hydraulic Fracture Treatment Process

The experiments discussed prior can yield valuable data. For example,the fracturability of mudstone can be predicted by interpreting the loadcurves from varying samples. The fracturability data assists incalculated pressure in flow rates during a hydraulic fracture treatmentprocess, such as the example illustrated later. The experimentsdiscussed prior can also be utilized for evaluating different chemicaltreatments. For example, a shale sample can be treated with a fluiddesigned to break-down kerogen. The treated sample can then befabricated into a micro-beam and tested to demonstrate the fluidseffects on kerogen. Such knowledge can improve the effectiveness ofhydraulic fracture treatments such as the example given in the followingparagraphs.

The kerogen content of different beam specimens in the previouslydiscussed experiments can be varied and the tensile test resultscompared directly. The beam specimen can even come from the same bulkshale sample, but taken from high, low, or intermediate kerogen contentregions. Without the kerogen, the beam will undergo brittle tensilefailure under load, with minimal tensile mode energy required to breakit. With kerogen, the energy required as well as its correlative tensilestrength will be much higher.

In compression, higher kerogen content will lead to lower compressivestrength. Therefore two pillars of equivalent size and dimension butdifferent kerogen content will yield differently under compressiveloads. Kerogen is understood to be at least 10 s time weaker than therock granular structure, depending on its maturity, in compression.Hydraulic fracturing is primarily a tensile failure of the rock in aMode I fracture propagation criteria, so the tensile properties(micro-cantilever beam tests) are the most relevant to fracturabilityconsiderations when it comes to optimizing hydraulic fracturing planningand execution.

FIG. 25 illustrates an example of a fracture treatment 10 for a well 12.The well 12 can be a reservoir or formation 14, for example, anunconventional reservoir in which recovery operations in addition toconventional recovery operations are practiced to recover trappedhydrocarbons. Examples of unconventional reservoirs include tight-gassands, gas and oil shales, coalbed methane, heavy oil and tar sands,gas-hydrate deposits, to name a few. In some implementations, theformation 14 includes an underground formation of naturally fracturedrock containing hydrocarbons (for example, oil, gas or both). Forexample, the formation 14 can include a fractured shale. In someimplementations, the well 12 can intersect other suitable types offormations 14, including reservoirs that are not naturally fractured inany significant amount.

The well 12 can include a well bore 20, casing 22 and well head 24. Thewell bore 20 can be a vertical or deviated bore. The casing 22 can becemented or otherwise suitably secured in the well bore 12. Perforations26 can be formed in the casing 22 at the level of the formation 14 toallow oil, gas, and by-products to flow into the well 12 and be producedto the surface 25. Perforations 26 can be formed using shape charges, aperforating gun or otherwise.

For the fracture treatment 10, a work string 30 can be disposed in thewell bore 20. The work string 30 can be coiled tubing, sectioned pipe orother suitable tubing. A fracturing tool 32 can be coupled to an end ofthe work string 30. Packers 36 can seal an annulus 38 of the well bore20 above and below the formation 14. Packers 36 can be mechanical, fluidinflatable or other suitable packers.

One or more pump trucks 40 can be coupled to the work string 30 at thesurface 25. The pump trucks 40 pump fracture fluid 58 down the workstring 30 to perform the fracture treatment 10 and generate the fracture60. The fracture fluid 58 can include a fluid pad, proppants and/or aflush fluid. The pump trucks 40 can include mobile vehicles, equipmentsuch as skids or other suitable structures. The fracturing fluid can bea cross-linked gel, linear gel, synthetic polymer gel, or slickwaterwith friction reducer. The fluid can be proppant-laden.

One or more instrument trucks 44 can also be provided at the surface 25.The instrument truck 44 can include a fracture control system 46 and afracture simulator 47. The fracture control system 46 monitors andcontrols the fracture treatment 10. The fracture control system 46 cancontrol the pump trucks 40 and fluid valves to stop and start thefracture treatment 10 as well as to stop and start the pad phase,proppant phase and/or flush phase of the fracture treatment 10. Thefracture control system 46 communicates with surface and/or subsurfaceinstruments to monitor and control the fracture treatment 10. In someimplementations, the surface and subsurface instruments may comprisesurface sensors 48, down-hole sensors 50 and pump controls 52.

A quantity of energy applied by the fracture control system 46 togenerate the fractures 60 in the reservoir or formation 14 can beaffected not only by the properties of the reservoir rock in theformation but also by the organic matter (for example, kerogen 75)intertwined within the rock matrix.

In one example, five micro-scale cantilever rock sample beams wereprepared, each with a longitudinal axis parallel to the bedding plane asshown in FIG. 26A. Six micro-scale cantilever beams were prepared, eachwith a longitudinal axis perpendicular to the bedding plane as shown inFIG. 26A. The nano-indenter tip force was applied perpendicular to thelongitudinal axis. FIG. 30 is a plot 3000 showing load versusdisplacement curves of the six micro-scale rock sample beams preparedwith longitudinal axes parallel to the bedding planes. FIG. 31 is a plot3100 showing load versus displacement curves of the five micro-scalerock sample beams prepared with longitudinal axes perpendicular to thebedding planes. The load versus displacement curves reveal that moreenergy was required to fail the beams of the plot 3000 than those of theplot 3100. Using the classical beam theory (Bernoulli's beam or Euler'sbeam), the Young's Modulus E was determined using load and displacementvalues from the linear elastic portion of the cantilever beam loadingcurves shown in plots 3000 and 3100.

TABLE 4 Beam dimensions and mechanical properties dL L_(b) b h I P w ETest (μm) (μm) (μm) (μm) (μm⁴) (μN) (μm) (GPa) 1 25.66 23.71 9.21 9.21599.6 423.4 200.1 17.6 2 25.64 23.72 8.83 8.63 472.9 240.6 200.0 12.7 324.78 22.12 8.51 9.41 590.9 359.9 200.0 13.0 4 17.79 16.69 6.53 6.94181.9 258.8 200.2 12.1 5 27.59 26.14 9.04 9.10 567.7 337.5 200.2 19.2 629.31 28.47 8.45 8.91 498.1 775.2 300.5 41.6 7 18.56 17.75 8.01 7.12240.9 765.5 300.2 21.1 8 28.75 26.17 9.77 8.16 442.4 573.4 300.0 29.6 929.26 28.76 8.84 9.45 621.7 540.2 300.3 23.5 10 28.79 27.63 9.44 8.98569.7 612.5 300.4 26.8 11 30.87 29.78 9.21 9.32 621.3 547.1 300.0 27.3

In Table 4, which shows beam dimensions and mechanical properties, dL isthe beam length, L_(b) is the bending length, b is the beam width, h isthe beam height, I is the moment of inertia, and finally P and w areload and displacement values taken from a point on the linear elasticloading portion of the load-displacement curve. The Young's modulus Ewas calculated from these values, where E1 is from the orientationrepresented by beams 1-5 and E3 is represented by the orientation inbeams E6-11. E1 and E3 can be selected, which is why there is a rangefrom 1.3 to 3. The results indicate notable anisotropy in the amount ofenergy required to fail the beam depending on the direction oforientation of the bedding planes relative to the direction of load. Thedegree of anisotropy is expected to vary from one shale to the next. Theenergies obtained can be used in modeling predictions of hydraulicfracturing in shale.

FIG. 27 is a flowchart that shows an example process 2700 fordetermining properties of a nano-scale rock sample. At 2702, anano-scale beam is cut and formed from a kerogen-rich retrievedreservoir rock. The nano-scale beam has a minimum dimension of at least100 nanometers (nm) and a maximum dimension of at most 1000 nm. Forexample, the cross-sectional dimension can be of the order of hundrednanometers. For TEM imaging, a minimum dimension of less than 100 nm isalso possible. In some implementations, nano-scale beams of the same ordifferent cross-sectional dimensions can be manufactured using a FIBdescribed earlier. Analyzing a composite material at a nano-scale levelcan enable identifying the representative elementary volume (REV) of thematerial. In granular composite materials, for example, shale, cement,or similar granular composite materials, the REV is an intrinsic lengthscale at and above which the material behaves mechanically as the bulkmaterial, and below which the material starts to behave as individualgranular components. The material behavior below the intrinsic lengthscale can depend upon the specific grains being analyzed.

At 2704, a tension test is performed on the nano-scale beam. Forexample, the tension test can be a cantilever test in which thenano-scale beam is loaded using a nano-indenter such as those describedearlier. At 2706, the tension test is imaged using a TEM. For example,the nano-scale beam and the nano-indenter can be positioned inside theTEM and TEM images can be captured while the tension test is inprogress. Because a TEM operates in transmission mode, it is possible tosee near atomic scale features. For example, in crystalline materials,features known as dislocations can possibly be identified and observedduring deformation and failure. During the in situ TEM beam testing ofshale, movement of dark features in certain grains during the imagingsuggest that dislocations may be moving or nucleating (or both). The TEMcan also be used to perform selected area diffraction (SAD) where adiffraction pattern of crystalline regions can be captured and analyzedto determine the crystallographic orientation of grains. Thisorientation is well known to influence the values of the variousmechanical properties, such as Young's modulus or the shear modules.

In some implementations, as an option, at 2705, the nano-scale beam canbe heated to study the effect of heat on the mechanical properties ofthe nano-scale beam. For example, heat can be applied to thenano-indenter tip or to the stage on which the nano-scale beam ispositioned or both. In some implementations, the tension test and theimaging can be performed without the application of heat to furtherstudy the effect of heat on the mechanical properties of the nano-scalebeam. A Hysitron Pi-85 Pico-indenter was used to load thenano-micro-beams under displacement control mode, at a rate of between 1nm/s and 100 nm/s, for example, between 5 nm/s and 20 nm/s, for example,10 nm/s. The indenter tip was placed at the end of the beam, centeredalong the y-axis. The indenter tip is a flat circular punch geometry,with a diameter of 5 μm or other, different geometry or diameter. Allloading experiments were performed in situ under the TEM, where loadingof the nano-scale beams continued until failure. For a material with REVin the micro- or hundreds of nano-scale range (for example, sourceshale), TEM in situ mechanical testing offers a way to probe theindividual composite material behavior at the REV level and even downfurther, allowing us to understand the material's behavior and response.

FIG. 28 is a Transmission Electron Microscope (TEM) image 2800 of anano-scale beam prior to failure. FIG. 29 is a TEM image 2900 of anano-scale beam after failure. The post-failure image 2900 shows atortuous crack across the beam support together with a conchoidal oreven tortuous fracture tip propagation, which indicates an initiation ofa fracture in a composite organic matter intertwined with minerals. Suchfracture propagations have no continual preference or alignment ofweakening planes.

At 2708, a material parameter of the nano-micro-scale beam is determinedbased on results of the tension test and images obtained.

The techniques described in this disclosure can be implemented using acement mixture, for example, a mixture of cement and an organic additivesuch as polymers or fibers. In other words, micro- and nano-scale beamscan be prepared and tested using nano-indentation experimentsimplemented in a SEM or TEM. FIG. 32 is a flowchart 3200 that shows anexample process for determining properties of a cement mixture. At 3202,a beam can be formed from a cement mixture. The cement mixture caninclude cement and an additive, for example, an organic additive such aspolymer or fibers. A maximum dimension of the beam can be at most 1000μm. In some implementations, the beam can be a nano-scale beam with amaximum dimension of at most 1000 nm. At 3204, a mechanical experimentcan be performed on the beam. The mechanical experiment can be a tensiontest (for example, a cantilever test) or a compression test. At 3206,the mechanical experiment can be imaged, for example, using a SEM orTEM. That is, the beam can be positioned inside the SEM or TEM and canbe imaged while the mechanical experiment is being performed. At 3208, amaterial parameter of the additive in the beam can be determined basedon results of the mechanical experiment and the images. In someimplementations, before forming the beam, the cement mixture can betreated with an additive, for example, an organic additive. An effect ofthe additive on the mechanical properties, for example, of the cementmixture, of the cement, or of the additive, can be determined based onthe results of the mechanical experiment and the images. The results canbe used to develop cement mixtures for specific cementing applications.For example, cement mixtures developed using the results of themechanical experiment and the images can be used in wellboreapplications, for example, to case all or portions of a wellbore.

Thus, particular implementations of the subject matter have beendescribed. Other implementations are within the scope of the followingclaims.

The invention claimed is:
 1. A method comprising: forming a nano-scalebeam from kerogen-rich reservoir rock, the nano-scale beam comprisingreservoir rock and kerogen having polymeric properties, wherein amaximum dimension of the nano-scale beam is at least 100 nanometer (nm)and at most 1000 nm; performing a tension test on the nano-scale beam;imaging the tension test using a transmission electron microscope (TEM),wherein the tension test is a cantilever test; determining a materialparameter of the kerogen in the nano-scale beam based on results of thetension test and images obtained responsive to the imaging; and applyingheat to the nano-scale beam while performing the cantilever test.
 2. Themethod of claim 1, wherein the material parameter of the kerogen in thenano-scale beam comprises a tensile strength of the nano-scale beam. 3.The method of claim 1, wherein performing the cantilever test comprisesapplying a force of the order of micro-Newtons on a free-end of thenano-scale beam, wherein determining the material parameter comprisesmeasuring a bending of the cantilever responsive to force.
 4. The methodof claim 3, further comprising applying the force at a rate ofdisplacement of substantially between 1 nm/s to 100 nm/s.
 5. The methodof claim 3, further comprising applying the load until the nano-scalebeam fails.
 6. The method of claim 1, wherein the force is a cantileverforce applied using a nano-indenter, wherein performing the cantilevertest comprises: applying the heat to the nano-indenter; and applying thecantilever force using the nano-indenter while applying heat to thenano-indenter.
 7. The method of claim 6, wherein applying heat to thenano-scale beam comprises applying the heat directly to the nano-scalebeam and to the nano-indenter.
 8. The method of claim 1, furthercomprising determining an effect of the heat applied to the nano-scalebeam on the material parameter of the kerogen in the nano-scale beam. 9.The method of claim 8, further comprising determining a mechanicalproperty profile of the kerogen-rich reservoir rock based on the effectof the heat applied to the nano-scale beam.
 10. The method of claim 1,wherein the nano-scale beam comprises a plurality of stacked shalebedding planes, wherein the tension test is performed either parallel toor perpendicular to the plurality of stacked shale bedding planes. 11.The method of claim 10, wherein performing the tension test parallel tothe plurality of stacked shale bedding planes comprises applying tensionin a direction that is perpendicular to a direction in which theplurality of stacked shale bedding planes are stacked.
 12. The method ofclaim 10, wherein performing the tension test perpendicular to theplurality of stacked shale bedding planes comprises applying tension ina direction that is parallel to a direction in which the plurality ofstacked shale bedding planes are stacked.